A fundamental problem with real time quantitative chemical analysis under the current state of technology is the non-linear analyte response that may occur when the composition of a sample's background matrix is altered. The inconsistencies that occur when investigating an analyte in different material matrices are generally referred to as matrix effects. For true quantitative analysis, calibration of the analytical tool is typically made with matrix matched samples, presenting limitations in performing in situ and real time analysis, and adding cost and complexity
Inductively-coupled plasma—atomic emission spectroscopy (ICP-AES) is a long-established analytical technique for elemental analysis. An induction coil is used to create a steady-state plasma. Materials to be analyzed (referred to as the analyte) are digested in acids, and then introduced into the ICP via a nebulizer. The nebulizer makes small droplets containing the acid solution and dissolved analyte species, which then flow into the hot plasma (T˜6,000-10,000 K), where the droplets are vaporized and the analyte species are dissociated to atoms and ions. The atoms and ions are excited within the hot plasma, resulting in atomic emission. The atomic emission is collected and analyzed (i.e., atomic emission spectroscopy), which forms the basis of ICP-AES as an analytical scheme. Typically, calibration solutions are also analyzed, enabling quantification of the analyte signal by direct comparison of atomic emission intensity. ICP-AES enjoys high sensitivity, but drawbacks include the relatively high equipment costs and the need for sample preparation (i.e. digestion).
Inductively-coupled plasma—mass spectrometry (ICP-MS) is widely viewed as an improvement to ICP-AES. Basically, with ICP-MS, all the above steps of ICP-AES are followed. However, at the exit of the inductively-coupled plasma, there is an inlet to a mass spectrometer (MS). Hence some atoms and ions are swept into the mass spectrometer instrumentation, where the mass/charge ratio is measured and quantified. With ICP-MS, the steps of digestion and nebulization, and then vaporization and dissociation to atoms and ions, are used to introduce the appropriate form of analyte (namely ions) into the mass spectrometer. Advantages of MS as the actual sensor, rather than AES, include increased sensitivity, as well as elimination of some optical interference that can occur with AES. Drawbacks of ICP-MS include relatively higher equipment costs and complexity as compared to ICP-AES, and the required sample preparation (i.e., digestion).
Laser ablation ICP-AES (LA-ICP-AES) and laser ablation ICP-MS (LA-ICP-MS) use a laser to ablate the sample. With laser-ablation, the analyte sample is placed in an ablation cell (small vessel with optical access via windows), and a pulsed laser beam is used to ablate a portion of the sample. The ablation plume (atoms, ions and small particles) is then passed into the ICP using a carrier gas, where all processes are as described above. In essence, the digestion and nebulization step is replaced with direct laser sampling via laser ablation. The advantages of LA are the elimination of time-consuming sample digestion, and the ability for micro-analysis and spatial mapping. The drawbacks include additional cost and complexity, as well as the potential for introduction of matrix effects during the laser ablation step. Matrix effects in general refer to conditions where the actual analyte signal depends on the material matrix from which the element in question originated. Ideally, the analyte signal (e.g., AES signal or MS signal) only depends on the quantity of the particular element of interest. If the analyte signal is influenced by the presence of other elements, that is a matrix effect. Matrix effects can originate in the ICP step. However, the laser ablation step can introduce additional matrix effects due to things like (1) non-stoichiometric ablation, in which the ablation plume elemental concentrations do not match the elemental concentrations of the original solid material, and (2) non-stoichiometric transport, in which the elements in the ablation plume are not transported equally to the ICP and/or to the MS.
Laser-induced breakdown spectroscopy (LIBS) is considered an analytical spectroscopy technique in which a pulsed laser beam is used to create a laser-induced plasma. The laser induced plasma can be formed in gases, in aerosols (mixtures of gas and suspended particles), on and in liquids, and directly on solid surfaces. The laser-induced plasma is characterized by temperatures of 10,000 to 40,000 K and is highly ionized. The laser-induced plasma serves two functions with the LIBS technique. First, the laser-induced plasma samples the analyte by vaporizing and dissociating the analyte species to atoms and ions. Second, the laser-induced plasma then excites the atoms and ions, and the resulting atomic emission is collected and analyzed. The elimination of sample preparation allowing rapid and direct analysis is generally extolled as an advantage of LIBS. The primary disadvantage of LIBS is the lack of precision and sensitivity as compared to other analytical schemes. The large background emission (continuum radiation) of the hot laser-induced plasmas can mask and reduce the signal-to-noise ratios of the atomic emission signal from the analyte species, resulting in a lack of sensitivity. A lack of precision can be considered to arise from the highly non-linear laser-material interactions that occur during plasma formation and analyte vaporization. The plasma formation and growth can be highly related to the laser-material interactions; hence, the resulting plasmas can show considerable variation in the temporal and spatial development, and in the temporal and spatial temperature and free electron density profiles. These items can directly affect the analyte sampling, vaporization, dissociation, and ionization processes, which then all affect the atomic emission signal. Because the laser beam functions as both the sampling laser and to create the analytical plasma, elimination of these effects is difficult if not possible.
An application of LIBS is the direct analysis of solid materials. This application takes advantage of LIBS to eliminate the need for sample preparation (e.g., digestion). However, due to the laser-sample coupling, matrix effects are generally present. Additional effects can result from the interference of the atmosphere above the sample. As the laser beam strikes the solid sample and begins to form the plasma, the growing plasma as well as the remaining laser pulse may interact with the atmosphere above the sample. In some cases, the gas above the surface can absorb considerable laser energy, thereby obscuring the targeted material. Such problems can lead to additional non-linearities and loss of analytical precision and accuracy. To help mitigate such problems, a dual-pulse LIBS methodology has been implicated. With dual-pulse LIBS, two laser beams are used, typically separated in time by nanoseconds to tens of microseconds. There are many arrangements, including having the two lasers either parallel to each other or orthogonal to each other. On advantage of dual-pulse LIBS is that the initial laser-induced plasma formed by the first laser can rarify the atmospheric gas above the sample surface, thereby allowing the second laser to couple better into the target (i.e., laser-surface coupling) rather than couple into the cover gas (i.e., laser-atmosphere coupling or plasma-shielding). This can increase the mass of analyte sampled. Another benefit associated with dual-pulse LIBS is increased sensitivity. For example, the second pulse can “re-heat” the plasma, or as noted above, better couple into the solid material, with the ultimate benefit of increased analyte signal response.